The solubility of most ions in solution decreases with a decrease in the temperature or pressure of the solution. If dissolved ions are present, near their saturation concentration in the solution, a slight reduction in the temperature or pressure can result in precipitation of a portion of these ions. The ions frequently combine and deposit as a scale on any solid surface with which they come into contact, such as a vessel or conduit in which the solution is confined. An example of such a solution is a geothermal brine.
Geothermal brines are used, among other things, for the generation of electric power. Typically, a geothermal brine, having a temperature above about 400.degree. F., is flashed to a lower pressure in one or more flashing stages to produce steam and a spent brine. The steam is used to drive a steam turbine-electric generator combination. The spent brine is filtered and returned to the geothermal aquifer via a dedicated brine injection well. Typically, the steam is condensed and placed in a holding pond until a sufficient quantity is accumulated for reinjection into a dedicated condensate injection well. The amount of brine requiring reinjection is typically in excess of about 6000 gallons per minute. The amount of steam condensate produced, which also requires disposal, amounts to about 200 gallons per minute. Formidable problems are encountered in handling and disposing of such large amounts of heavily contaminated and highly saline geothermal liquids.
One of the more serious problems, encountered in using a geothermal brine for producing electric power, results from scaling and deposition of solids in the equipment used to confine the brine. A typical geothermal brine has been confined in a subterranean reservoir for an extraordinarily long period of time at elevated temperatures. As a result, large amounts of minerals have been leached from the reservoir into the brine. Typically, salts and oxides of heavy metals such as lead, zinc, iron, silver, cadmium, molybdenum, manganese and even gold are found in geothermal brines. Other more common minerals, such as calcium and sodium, also are dissolved in the brine, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide and methane. An especially troublesome component of the brine is silica.
All of these components tend to precipitate out at almost every stage of brine processing. Even when the brine has completed its passage through a plant, it will contain a sufficient concentration of these components to eventually result in plugging of the injection wells used to return the brine and condensate to the geothermal aquifer.
In a typical geothermal plant, the steam condensate is sent from the condenser to a holding pond at a rate of approximately 200 gallons per minute. The rate will vary throughout the year, with a higher discharge rate occurring during the cool winter months and a lower discharge rate occurring during the hot summer months. In addition, blow-down from the cooling towers also is discharged to the holding pond at periodic intervals. The condensate is held in the holding pond for a sufficient time to permit most of the sulfides and sulfites contained therein to oxidize to sulfates. The sulfates are less likely to form scales which could result in plugging of the injection well.
During operation of a geothermal-electric power plant, a certain amount of brine also is introduced into the holding pond. The brine generally results from spills, brine which is bypassed to the pond during startup, and a certain amount of wash water from brine filters. It has been found that the pond rapidly fills with a precipitate comprised of metal carbonates, hydroxides and sulfides. In an actual operating plant, the pond was substantially filled with a sludge of such precipitates after only seven years of operation. The sludge has no appreciable value. Further, traces of heavy metals could render the sludge a hazardous waste and necessitate its disposal in a site suitable for such a waste.
Obviously, it would be advantageous if the formation of the sludge could be eliminated or its disposal cost substantially mitigated. It would be an even greater advantage if the composition of the sludge could be converted to one having a commercial value.